Rectifier circuits and corresponding methods for rfid devices

ABSTRACT

There is described a rectifier circuit for providing and limiting a supply voltage to an RFID tag, the circuit including a pair of antenna input terminals configured to receive an input signal from an RFID tag antenna. A plurality of charge pump stages are coupled in cascade in such a way that an input terminal of a first charge pump stage in the cascade is connected to ground and an input terminal of each subsequent charge pump stage in the cascade is coupled to an output terminal of the preceding charge pump stage in the cascade. A control logic is configured to select the output terminal of one charge pump stage among the plurality of charge pump stages to provide the supply voltage. Furthermore, an RFID tag and a method of providing and limiting a supply voltage to an RFID tag are described.

TECHNICAL FIELD

The present disclosure relates to the field of RF communication devices.More specifically, the present disclosure relates to rectifier circuitsand methods for providing and limiting a supply voltage to an RFID tag.Furthermore, the present disclosure relates to an RFID tag comprising acorresponding rectifier circuit.

ART BACKGROUND

State of the art passive UHF RFID tags use a charge pump for generatingthe main power supply, i.e. by converting the AC voltage from theantenna into a DC supply voltage. The output voltage of this rectifieris strongly dependent on input power and impedance matching. To preventphysical damage to the electronic components in RFID tags, state of theart implementations utilize a charge pump limiter that clamps the outputvoltage of the rectifier to a certain level. However, this approachcauses the tag impedance to change and thus has a strong de-tuningeffect which may lead to significant reductions in the return linkperformance, i.e. a reduced strength of the signal received by thereader device.

There may thus be a need for a way of providing a rectifier circuit thathas efficient voltage limitation but without the aforementioneddrawbacks.

SUMMARY

This need may be met by the subject matter according to the independentclaims. Advantageous embodiments of the present disclosure are set forthin the dependent claims.

According to a first aspect, there is provided a rectifier circuit forproviding and limiting a supply voltage to an RFID tag, the circuitcomprising a pair of antenna input terminals configured to receive aninput signal from an RFID tag antenna; a plurality of charge pumpsstages, each charge pump stage comprising a pair of charging terminalsconnected to the pair of antenna input terminals, an input terminal, andan output terminal, wherein the plurality of charge pump stages arecoupled in cascade in such a way that the input terminal of a firstcharge pump stage in the cascade is connected to ground and the inputterminal of each subsequent charge pump stage in the cascade is coupledto the output terminal of the preceding charge pump stage in thecascade; and a control logic configured to select the output terminal ofone charge pump stage among the plurality of charge pump stages toprovide the supply voltage.

This aspect is based on the idea that the desired supply voltage isselectively taken from an output terminal of one of a plurality ofcharge pump stages coupled in a cascade-like manner. Each single chargepump stage in the cascade is connected to the antenna input (to becharged by an input signal received from an RFID antenna) such that thecascade of charge pump stages presents a constant impedance independentof which particular charge pump stage is selected for providing thesupply voltage at a given time. Hence, the power supply circuit iscapable of providing a desired and limited supply voltage whilemaintaining an essentially constant impedance. As the charge pump stagesare coupled in cascade, none of the individual charge pump stages isexposed to the total supply voltage. In other words, the charge pumpstages are floating. Due to the constant impedance of the circuit,decoupling effects can be avoided, and a correspondingly efficientreturn link performance is assured.

According to an embodiment, each of the plurality of charge pump stagesis configured to provide a predetermined voltage difference between itsrespective output terminal and input terminal.

In other words, the voltage difference between output and input terminalof any charge pump stage in the cascade is essentially the same. Thus,if the predetermined voltage difference is positive, e.g. 0.2 V, thenthe voltage at the output terminal of the fifth charge pump stage in thecascade (where the first charge pump stage in the cascade has its inputterminal connected to ground) will be equal to 1.0 V. Similarly, if thevoltage difference is negative, e.g. −0.2 V, then the voltage at theoutput terminal of the fifth charge pump stage in the cascade will beequal to −1.0 V. The predetermined voltage difference is dependent onthe power of the received input signal.

According to a further embodiment, the control logic is configured toselect the output terminal of the charge pump among the plurality ofcharge pump stages at which output terminal the voltage is closest toand not above an upper threshold voltage.

In other words, the control logic selects the charge pump outputterminal that has voltage as close as possible to the upper thresholdvoltage without exceeding said upper threshold voltage.

According to a further embodiment, the control logic is configured toinitially select the output terminal of the last charge pump in thecascade, compare the voltage at the initially selected output terminalwith an upper threshold voltage, and if said voltage exceeds the upperthreshold voltage, select the output terminal of the one of thepreceding charge pumps in the cascade, at which output terminal thevoltage is closest to and not above said upper threshold voltage.

In this embodiment, the control logic starts with the output terminal ofthe last charge pump in the cascade, i.e. the highest voltage available,and compares this voltage with the upper threshold voltage. Thecomparison may be done utilizing a comparator circuit. If thecorresponding voltage exceeds the upper threshold voltage, the controllogic moves backwards through the cascade of charge pump stages until anoutput terminal is found that has a voltage below or equal to the upperthreshold voltage.

According to a further embodiment, the control logic comprises aplurality of controllable switches, each switch being arranged betweenthe output terminal of one of the plurality of charge pump stages and arectifier output terminal.

According to a further embodiment, the control logic is configured toclose and open the plurality of switches such that the selected outputterminal is connected to the rectifier output terminal.

According to a further embodiment, the pair of antenna input terminalscomprises a positive input terminal and a negative input terminal, thepair of charging terminals of each charge pump stage comprises apositive charging terminal and a negative charging terminal, and for anytwo neighboring charge pump stages in the cascade, the positive chargingterminal of one of the two neighboring charge pump stages is connectedto the positive input terminal, the positive charging terminal of theother one of the two neighboring charge pump stages is connected to thenegative input terminal, the negative charging terminal of the one ofthe two neighboring charge pumps is connected to the negative inputterminal, and the negative charging terminal of the other one of the twoneighboring charge pump stages is connected to the positive inputterminal.

In other words, the charge pumps in the cascade are arranged withalternating polarity of the charge terminals relative to the antennainput terminals such that neighboring charge pump stages receivealternating phases at their respective charging terminals. Thereby,efficient charging can be achieved for input signals with highfrequency.

According to a second aspect, there is provided an RFID tag comprisingan RFID tag antenna, an RFID tag processing circuit, and a rectifiercircuit configured to provide and limit a supply voltage to the RFID tagprocessing circuit. The rectifier circuit comprises a pair of antennainput terminals coupled to receive an input signal from the RFID tagantenna, a plurality of charge pump stages, each charge pump stagecomprising a pair of charging terminals connected to the pair of antennainput terminals, an input terminal, and an output terminal, wherein theplurality of charge pump stages are coupled in cascade in such a waythat the input terminal of a first charge pump stage in the cascade isconnected to ground and the input terminal of each subsequent chargepump stage in the cascade is coupled to the output terminal of thepreceding charge pump stage in the cascade, and a control logicconfigured to select the output terminal of one charge pump stage amongthe plurality of charge pump stages to provide the supply voltage.

This aspect is essentially based on the same idea as the first aspectdiscussed above and provides an RFID tag utilizing the rectifier circuitaccording to the first aspect. Hence, the supply voltage for the RFIDtag processing circuit is selectively taken from an output terminal ofone of a plurality of charge pump stages coupled in a cascade-likemanner. Each single charge pump stage in the cascade is connected to theantenna input (to be charged by an input signal received from the RFIDantenna) such that the cascade of charge pumps presents a constantimpedance independent of which particular charge pump stage is selectedfor providing the supply voltage at a given time. Hence, the powersupply circuit is capable of providing a suitable and limited supplyvoltage while maintaining an essentially constant input impedance. Asthe charge pump stages are coupled in cascade, none of the individualcharge pump stages is exposed to the total supply voltage. In otherwords, the charge pump stages are floating. Due to the constant inputimpedance of the circuit, decoupling effects can be avoided, and acorrespondingly efficient return link performance is assured.

According to a further embodiment, each of the plurality of charge pumpstages are configured to provide a predetermined voltage differencebetween its respective output terminal and input terminal.

In other words, the voltage difference between output and input terminalof any charge pump in the cascade is essentially the same. Thus, if thepredetermined voltage difference is positive, e.g. 0.2 V, then thevoltage at the output terminal of the fifth charge pump stage in thecascade (where the first charge pump stage in the cascade has its inputterminal connected to ground) will be equal to 1.0 V. Similarly, if thevoltage difference is negative, e.g. −0.2 V, then the voltage at theoutput terminal of the fifth charge pump stage in the cascade will beequal to −1.0 V. The predetermined voltage difference is dependent onthe power of the received input signal.

According to a further embodiment, the control logic is configured toselect the output terminal of the charge pump stage among the pluralityof charge pump stages at which output terminal the voltage is closest toand not above an upper threshold voltage.

In other words, the control logic selects the charge pump stage outputterminal that has voltage as close as possible to the upper thresholdvoltage without exceeding said upper threshold voltage.

According to a further embodiment, the control logic is configured toinitially select the output terminal of the last charge pump stage inthe cascade, compare the voltage at the initially selected outputterminal with an upper threshold voltage, and if said voltage exceedsthe upper threshold voltage, select the output terminal of the one ofthe preceding charge pump stages in the cascade, at which outputterminal the voltage is closest to and not above said upper thresholdvoltage.

In this embodiment, the control logic starts with the output terminal ofthe last charge pump stage in the cascade, i.e. the highest voltageavailable, and compares this voltage with the upper threshold voltage.The comparison may be done utilizing a comparator circuit. If thecorresponding voltage exceeds the upper threshold voltage, the controllogic moves backwards through the cascade of charge pump stages until anoutput terminal is found that has a voltage below or equal to the upperthreshold voltage.

According to a further embodiment, the control logic comprises aplurality of controllable switches, each switch being arranged betweenthe output terminal of one of the plurality of charge pump stages and arectifier output terminal.

According to a further embodiment, the control logic is configured toclose and open the plurality of switches such that the selected outputterminal is connected to the rectifier output terminal.

According to a further embodiment, the pair of antenna input terminalscomprises a positive input terminal and a negative input terminal, thepair of charging terminals of each charge pump stage comprises apositive charging terminal and a negative charging terminal, and for anytwo neighboring charge pump stages in the cascade, the positive chargingterminal of one of the two neighboring charge pump stages are connectedto the positive input terminal, the positive charging terminal of theother one of the two neighboring charge pump stages are connected to thenegative input terminal, the negative charging terminal of the one ofthe two neighboring charge pump stages are connected to the negativeinput terminal, and the negative charging terminal of the other one ofthe two neighboring charge pump stages are connected to the positiveinput terminal.

In other words, the charge pump stages in the cascade are arranged withalternating polarity of the charge terminals relative to the antennainput terminals. Thereby, efficient charging can be achieved for inputsignals with high frequency.

According to a third aspect, there is provided a method of providing andlimiting a supply voltage to an RFID tag, the method comprisingproviding a pair of antenna input terminals configured to receive aninput signal from an RFID tag antenna, providing a plurality of chargepump stages, each charge pump stage comprising a pair of chargingterminals connected to the pair of antenna input terminals, an inputterminal, and an output terminal, wherein the plurality of charge pumpstages are coupled in cascade in such a way that the input terminal of afirst charge pump stage in the cascade is connected to ground and theinput terminal of each subsequent charge pump stage in the cascade iscoupled to the output terminal of the preceding charge pump stage in thecascade, and selecting the output terminal of one charge pump stageamong the plurality of charge pump stages to provide the supply voltage.

This aspect is essentially based on the same idea as the first andsecond aspects described above and provides a corresponding method ofproviding a supply voltage to an RFID tag according to which the supplyvoltage for the RFID tag is selectively taken from an output terminal ofone of a plurality of charge pump stages coupled in a cascade-likemanner. Each single charge pump stage in the cascade is connected to theantenna input (to be charged by an input signal received from the RFIDantenna) such that the cascade of charge pump stages presents a constantimpedance independent of which particular charge pump stage is selectedfor providing the supply voltage at a given time. Hence, a suitable andlimited supply voltage can be provided while maintaining an essentiallyconstant impedance. As the charge pump stages are coupled in cascade,none of the individual charge pump stages is exposed to the total supplyvoltage. In other words, the charge pump stages are floating. Due to theconstant input impedance of the circuit, decoupling effects can beavoided, and a correspondingly efficient return link performance isassured.

According to a further embodiment, the selecting step comprisesselecting the output terminal of the charge pump stage among theplurality of charge pump stages at which output terminal the voltage isclosest to and not above an upper threshold voltage.

In other words, the charge pump stage output terminal that has voltageas close as possible to the upper threshold voltage without exceedingsaid upper threshold voltage is selected.

According to a further embodiment, the selecting step comprisesinitially selecting the output terminal of the last charge pump stage inthe cascade, comparing the voltage at the initially selected outputterminal with an upper threshold voltage, and if said voltage exceedsthe upper threshold voltage, selecting the output terminal of the one ofthe preceding charge pump stages in the cascade, at which outputterminal the voltage is closest to and not above said upper thresholdvoltage.

In this embodiment, the selection step starts with the output terminalof the last charge pump stage in the cascade, i.e. the highest voltageavailable, and compares this voltage with the upper threshold voltage.If the corresponding voltage exceeds the upper threshold voltage, theselection process moves backwards through the cascade of charge pumpstages until an output terminal is found that has a voltage below orequal to the upper threshold voltage.

According to a further embodiment, the selecting step comprisesoperating a plurality of controllable switches, each switch beingarranged between the output terminal of one of the plurality of chargepump stages and a rectifier output terminal.

According to a further embodiment, the selecting step comprises closingand opening the plurality of switches such that the selected outputterminal is connected to the rectifier output terminal.

It should be noted that exemplary embodiments have been described withreference to different subject matters. In particular, some embodimentshave been described with reference to method type claims whereas otherembodiments have been described with reference to apparatus type claims.However, a person skilled in the art will gather from the above and thefollowing description that, unless otherwise indicated, in addition toany combination of features belonging to one type of subject matter alsoany combination of features relating to different subject matters, inparticular a combination of features of the method type claims andfeatures of the apparatus type claims, is also disclosed with thisdocument.

The aspects defined above and further aspects of the present disclosurewill be apparent from the examples of embodiment to be describedhereinafter and are explained with reference to the examples ofembodiment. Aspects of the present disclosure will be described in moredetail hereinafter with reference to examples of embodiment to which thepresent disclosure is, however, not limited.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a block diagram of a rectifier circuit for an RFID tagaccording to an embodiment.

FIG. 2 shows a circuit diagram of a rectifier circuit for an RFID tagaccording to an embodiment.

FIG. 3 shows a flowchart of a method of providing and limiting a supplyvoltage to an RFID tag in accordance with an embodiment.

DETAILED DESCRIPTION

The illustration in the drawing is schematic. It is noted that indifferent figures, similar or identical elements are provided with thesame reference signs or with reference signs, which differ only withinthe first digit.

FIG. 1 shows a block diagram of a rectifier circuit 100 for an RFID tagaccording to an embodiment. The rectifier circuit 100 comprises a pairof antenna input terminals 102, 104 configured to receive an inputsignal from an RFID tag antenna (not shown), a plurality of charge pumpstages 1101, 1102, 1103, 110 n coupled in cascade, and a control logic120.

Each of the charge pump stages 1101, 1102, 1103, 110 n comprises a pairof charging terminals connected to the pair of antenna input terminals102, 104, an input terminal, and an output terminal. More specifically,charge pump stage 1101 comprises charging terminal 1121 connected toantenna input terminal 102, charging terminal 1141 connected to antennainput terminal 104, input terminal 1161, and output terminal 1181.Charge pump stage 1102 comprises charging terminal 1122 connected toantenna input terminal 102, charging terminal 1142 connected to antennainput terminal 104, input terminal 1162, and output terminal 1182.Charge pump stage 1103 comprises charging terminal 1123 connected toantenna input terminal 102, charging terminal 1143 connected to antennainput terminal 104, input terminal 1163, and output terminal 1183.Charge pump stage 110 n comprises charging terminal 112 n connected toantenna input terminal 102, charging terminal 114 n connected to antennainput terminal 104, input terminal 116 n, and output terminal 118 n.

The charge pump stages 1101, 1102, 1103, 110 n are coupled in cascade insuch a way that the input terminal 1161 of the first charge pump stage1101 in the cascade is connected to ground and the input terminal ofeach subsequent charge pump stage in the cascade is coupled to theoutput terminal of the preceding charge pump stage in the cascade. Morespecifically, the input terminal 1162 of the second charge pump stage1102 is connected to the output terminal 1181 of the first charge pumpstage 1101, the input terminal 1163 of the third charge pump stage 1103is connected to the output terminal 1182 of the second charge pump stage1102, and the input terminal 116 n of the last (nth) charge pump stage110 n in the cascade is connected to the output terminal of thenext-to-last charge pump stage (not shown). Hence, all charge pumpstages 1101, 1102, 1103, 110 n in the cascade have their respectivecharging terminals connected to the antenna input terminals 102, 104 andonly “see” the voltage difference between their respective output andinput terminals. This voltage difference is preferably the same (orclose to being the same) across each charge pump stage in the cascade,such that the output voltage increments with essentially the same amountfor each charge pump stage in the cascade. The cascade of charge pumpstages 1101, 1102, 1103, 110 n is also referred to as a cascade offloating charge pump stages.

The power supply circuit further comprises a control logic 120configured to select the output terminal 1181, 1182, 1183, 118 n of oneof the charge pump stages 1101, 1102, 1103, 110 n to provide the supplyvoltage 140 for the RFID tag circuitry (not shown). In this embodiment,the selection is done by providing control signals 1221, 1222, 1223, 122n from the control logic 120 to each of a plurality of switches 1301,1302, 130 n in order to close a connection between the rectifier outputterminal 140 and a single one of the charge pump stage output terminals,such as output terminal 118 n of the last charge pump stage 110 n in thecascade. The selection of the particular output terminal may involvecomparing (e.g. utilizing a comparator circuit) the correspondingvoltage to an upper threshold voltage and, if necessary, stepping backthrough the cascade until an output voltage is found that does notexceed the upper threshold voltage. Once the proper charge pump stageoutput terminal is found, the corresponding supply voltage 140 isprovided to the RFID tag circuitry which can then send its response tothe reader device from which the input signal originated. For the rareand unlikely case, that the voltage at the output terminal 1181 of thefirst charge pump stage is above the threshold, the rectifier circuitmay comprise a conventional voltage limiter (not shown) to make surethat the rectifier output voltage is below the upper threshold voltage.Only if this limiter is activated, the circuit impedance will change andcorresponding detuning and reduction in return link performance mayoccur.

FIG. 2 shows a circuit diagram of a rectifier circuit 200 for an RFIDtag according to an embodiment. The overall structure of rectifiercircuit 200 is similar to the circuit 100 shown in FIG. 1 and discussedabove. More specifically, the circuit 200 comprises a cascade of chargepump stages 2101, 2102, 2103, 210 n, each comprising two capacitors andfour transistors. More specifically, the first charge pump stage 2101comprises capacitors C11, C21, NMOS transistors T11, T21, and PMOStransistors T31, T41. Similarly, the second charge pump stage 2102comprises capacitors C12, C22, NMOS transistors T12, T22, and PMOStransistors T32, T42, while the third charge pump stage 2103 comprisescapacitors C13, C23, NMOS transistors T13, T23, and PMOS transistorsT33, T43 (not shown). Finally, the last (n-th) charge pump stage 210 ncomprises capacitors C1 n, C2 n, NMOS transistors T1 n, T2 n (notshown), and PMOS transistors T3 n, T4 n. Preferably, all capacitors havethe same capacitance, and the corresponding transistors in the chargepump stages are identical. Referring to the first charge pump stage 2101as an example, the first capacitor C11 is coupled between the firstcharging terminal 2121 and the gates of transistors T21 and T41, whilethe second capacitor C21 is coupled between the second charging terminal2141 and the gates of transistors T11 and T31.

The first charge pump stage 2101 comprises a first charging terminal2121 and a second charging terminal 2141, the second charge pump stage2102 comprises a first charging terminal 2122 and a second chargingterminal 2142, the third charge pump stage 2103 comprises a firstcharging terminal 2123 and a second charging terminal 2143, and the n-thcharge pump stage 210 n comprises a first charging terminal 212 n and asecond charging terminal 214 n. As can be seen in FIG. 2, the firstcharging terminals 2121, 2123 of the first and third charge pump stages2101, 2103 (and other charge pump stages located in an uneven numberedposition in the cascade) are located in the upper part of the circuit,while the first charging terminal 2122 of the second charge pump stage2102 (and other charge pump stages located in an even numbered positionin the cascade) are located in the lower part of the circuit. Similarly,the second charging terminals 2141, 2143 of the first and third chargepump stages 2101, 2103 (and other charge pump stages located in anuneven numbered position in the cascade) are located in the lower partof the circuit, while the second charging terminal 2122 of the secondcharge pump stage 2102 (and other charge pump stages located in an evennumbered position in the cascade) are located in the upper part of thecircuit. Hence, since the first charging terminals are all connected toone of the input terminals (not shown) while all second chargingterminals are connected to the other one of the input terminals (notshown), neighboring charge pump stages are operating essentially 180°out of phase.

The first charge pump stage 2101 comprises output terminals 2181, thesecond charge pump stage 2102 comprises output terminals 2182, and thelast charge pump stage 210 n comprises output terminals 218 n. Controllogic and switches (not shown) similar to the arrangement shown in FIG.1 and discussed above are configured to select and connect the outputterminals 2181, 2182, . . . 218 n of one of the charge pump stages inthe cascade to provide the corresponding supply voltage to the RFID tagcircuitry as previously discussed.

FIG. 3 shows a flowchart of a method 300 of providing a supply voltageto an RFID tag in accordance with an embodiment.

The method 300 begins at 310 by providing a pair of antenna inputterminals configured to receive an input signal from an RFID tagantenna.

Then, at 320, a plurality of charge pump stages is provided. Each chargepump stage comprises a pair of charging terminals connected to the pairof antenna input terminals, an input terminal, and an output terminal.The plurality of charge pump stages are coupled in cascade in such a waythat the input terminal of a first charge pump stage in the cascade isconnected to ground and the input terminal of each subsequent chargepump stage in the cascade is coupled to the output terminal of thepreceding charge pump stage in the cascade. The cascade of charge pumpstages is formed to have a certain and essentially constant impedance inorder to optimize matching and tuning.

At 330, the output terminal of one charge pump stage among the pluralityof charge pump stages is selected to provide the supply voltage.

It is noted that, unless otherwise indicated, the use of terms such as“upper”, “lower”, “left”, and “right” refers solely to the orientationof the corresponding drawing.

It is noted that the term “comprising” does not exclude other elementsor steps and that the use of the articles “a” or “an” does not exclude aplurality. Also, elements described in association with differentembodiments may be combined. It should also be noted that referencesigns in the claims should not be construed as limiting the scope of theclaims.

1-15. (canceled)
 16. A rectifier circuit for providing and limiting asupply voltage to an RFID tag, the circuit comprising: a pair of antennainput terminals configured to receive an input signal from an RFID tagantenna; a plurality of charge pump stages, each charge pump stagecomprising a pair of charging terminals connected to the pair of antennainput terminals, an input terminal, and an output terminal, wherein theplurality of charge pump stages are coupled in cascade in such a waythat the input terminal of a first charge pump stage in the cascade isconnected to ground and the input terminal of each subsequent chargepump stage in the cascade is coupled to the output terminal of thepreceding charge pump stage in the cascade; and a control logicconfigured to select the output terminal of one charge pump stage amongthe plurality of charge pump stages to provide the supply voltage. 17.The circuit according to claim 16, wherein each of the plurality ofcharge pump stages is configured to provide a predetermined voltagedifference between its respective output terminal and input terminal.18. The circuit according to claim 16, wherein the control logic isconfigured to select the output terminal of the charge pump stage amongthe plurality of charge pump stages at which output terminal the voltageis closest to and not above an upper threshold voltage.
 19. The circuitaccording to claim 16, wherein the control logic is configured toinitially select the output terminal of the last charge pump stage inthe cascade, compare the voltage at the initially selected outputterminal with an upper threshold voltage, and if said voltage exceedsthe upper threshold voltage, select the output terminal of the one ofthe preceding charge pump stages in the cascade, at which outputterminal the voltage is closest to and not above said upper thresholdvoltage.
 20. The circuit according to claim 16, wherein the controllogic comprises a plurality of controllable switches, each switch beingarranged between the output terminal of one of the plurality of chargepump stages and a rectifier output terminal.
 21. The circuit accordingto claim 20, wherein the control logic is configured to close and openthe plurality of switches such that the selected output terminal isconnected to the rectifier output terminal.
 22. The circuit according toclaim 16, wherein the pair of antenna input terminals comprises apositive input terminal and a negative input terminal, the pair ofcharging terminals of each charge pump stage comprises a positivecharging terminal and a negative charging terminal, and for any twoneighboring charge pump stages in the cascade, the positive chargingterminal of one of the two neighboring charge pump stage is connected tothe positive input terminal, the positive charging terminal of the otherone of the two neighboring charge pump stage is connected to thenegative input terminal, the negative charging terminal of the one ofthe two neighboring charge pump stages is connected to the negativeinput terminal, and the negative charging terminal of the other one ofthe two neighboring charge pump stages is connected to the positiveinput terminal.
 23. An RFID tag comprising: an RFID tag antenna; an RFIDtag processing circuit; and a rectifier circuit configured to provideand limit a supply voltage to the RFID tag processing circuit, whereinthe rectifier circuit comprises: a pair of antenna input terminalscoupled to receive an input signal from the RFID tag antenna; aplurality of charge pump stages, each charge pump stage comprising apair of charging terminals connected to the pair of antenna inputterminals, an input terminal, and an output terminal, wherein theplurality of charge pump stages are coupled in cascade in such a waythat the input terminal of a first charge pump stage in the cascade isconnected to ground and the input terminal of each subsequent chargepump stage in the cascade is coupled to the output terminal of thepreceding charge pump stage in the cascade; and a control logicconfigured to select the output terminal of one charge pump stage amongthe plurality of charge pump stages to provide the supply voltage. 24.The RFID tag according to claim 23, wherein each of the plurality ofcharge pump stages is configured to provide a predetermined voltagedifference between its respective output terminal and input terminal.25. The RFID tag according to claim 23, wherein the control logic isconfigured to select the output terminal of the charge pump stage amongthe plurality of charge pump stages at which output terminal the voltageis closest to and not above an upper threshold voltage.
 26. The RFID tagaccording to claim 23, wherein the control logic is configured toinitially select the output terminal of the last charge pump stage inthe cascade, compare the voltage at the initially selected outputterminal with an upper threshold voltage, and if said voltage exceedsthe upper threshold voltage, select the output terminal of the one ofthe preceding charge pump stages in the cascade, at which outputterminal the voltage is closest to and not above said upper thresholdvoltage.
 27. The RFID tag according to claim 23, wherein the controllogic comprises a plurality of controllable switches, each switch beingarranged between the output terminal of one of the plurality of chargepump stages and a rectifier terminal.
 28. The RFID tag according toclaim 27, wherein the control logic is configured to close and open theplurality of switches such that the selected output terminal isconnected to the rectifier output terminal.
 29. The RFID tag accordingto claim 23, wherein the pair of antenna input terminals comprises apositive input terminal and a negative input terminal, the pair ofcharging terminals of each charge pump stage comprises a positivecharging terminal and a negative charging terminal, and for any twoneighboring charge pump stages in the cascade, the positive chargingterminal of one of the two neighboring charge pump stages is connectedto the positive input terminal, the positive charging terminal of theother one of the two neighboring charge pump stages is connected to thenegative input terminal, the negative charging terminal of the one ofthe two neighboring charge pump stages is connected to the negativeinput terminal, and the negative charging terminal of the other one ofthe two neighboring charge pump stages is connected to the positiveinput terminal.
 30. A method of providing and limiting a supply voltageto an RFID tag, the method comprising: providing a pair of antenna inputterminals configured to receive an input signal from an RFID tagantenna; providing a plurality of charge pump stages, each charge pumpstage comprising a pair of charging terminals connected to the pair ofantenna input terminals, an input terminal, and an output terminal,wherein the plurality of charge pump stages are coupled in cascade insuch a way that the input terminal of a first charge pump stage in thecascade is connected to ground and the input terminal of each subsequentcharge pump stage in the cascade is coupled to the output terminal ofthe preceding charge pump stage in the cascade; and selecting the outputterminal of one charge pump stage among the plurality of charge pumpstages to provide the supply voltage.
 31. The method according to claim30, wherein the selecting step comprises selecting the output terminalof the charge pump stage among the plurality of charge pump stages atwhich output terminal the voltage is closest to and not above an upperthreshold voltage.
 32. The method according to claim 30, wherein theselecting step comprises initially selecting the output terminal of thelast charge pump stage in the cascade, comparing the voltage at theinitially selected output terminal with an upper threshold voltage, andif said voltage exceeds the upper threshold voltage, selecting theoutput terminal of the one of the preceding charge pump stages in thecascade, at which output terminal the voltage is closest to and notabove said upper threshold voltage.
 33. The method according to claim30, wherein the selecting step comprises operating a plurality ofcontrollable switches, each switch being arranged between the outputterminal of one of the plurality of charge pump stages and a rectifieroutput terminal.
 34. The method according to claim 33, wherein theselecting step comprises closing and opening the plurality of switchessuch that the selected output terminal is connected to the rectifieroutput terminal.